Abstract: Chemical
shielding anisotropy tensors have been determined for all twenty-seven
characteristic conformers of For-L-Val-NH2 using the GIAO-RHF formalism
with the 6-31 + G* and TZ2P basis sets. The individual chemical shifts
and their conformational averages have been compared to their experimental
counterparts taken from the BioMagnetic Resonance Bank (BMRB). At the highest
level of theory applied, for all nuclei but the amide proton, deviations
between statistically averaged theoretical and experimental chemical shifts
are as low as 1-3%. Correlated chemical shift plots of selected nuclei,
as function of the respective phi, psi, chi1, and chi2 torsional angles,
have been generated. On two-dimensional chemical shift-chemical shift plots,
for example, 1HNH-15NNH and
15NNH-13Ca,
regions corresponding to major conformational clusters have been identified,
providing a basis for the quantitative identification of conformers from
NMR shift data. Experimental NMR resonances of nuclei of valine residues
have been deduced from 18 selected proteins, resulting in 93 1Ha-13Ca
chemical shift pairs. These experimental results have been compared to
relevant ab initio values revealing remarkable correlation between the
two sets of data. Correlations of 1Ha
and 13Ca
values with backbone conformational parameters (phi and psi) have also
been found for all pairs (e.g. 1Ha/phi
and 13Ca/phi)
but 1Ha/psi.
Overall, the appealing idea of establishing backbone folding of proteins
by employing chemical shift information alone, obtained from selected multiple-pulse
NMR experiments (e.g. 2D-HSQC, 2D-HMQC, and 3D-HNCA), has received further
support.